It is possible to use a focus electrode as a gating electrode to constitute an electronic shutter. Another known technique is to include a single grid electrode in the region between the cathode and the focus electrode and such a grid electrode can be used for gating likewise. However, the use of a focus electrode or a grid electrode in this manner for gating requires that the pulse applied to the electrode to provide shuttering action must have a nearly perfect "rectangular" shape. During the rise and fall times of the pulse, the tube is effectively out of focus and a severe loss of resolution occurs if the rise and fall times occupy a substantial proportion of the total duration of the pulse. Moreover, if the top of the applied pulse exhibits substantial overshoot or ripple, the tube does not remain in optimum focus for the duration of shuttering. Another effect is that the electron-optical magnification of the tube varies as the focus or grid potential changes and so the use of the focus electrode or the single grid electrode for shuttering causes smearing of the image on the screen and has a disastrous effect on the contrast of the image.
It is known from GB-PS No.-1458399 to provide in a streaking or framing image tube a focussing electrode assembly constituted by a conical focussing electrode and, between the photo cathode and the conical focussing electrode, two electrodes constituted by meshes. The first of these electrodes is used as an accelerating electrode. The second electrode recieves an applied potential which is positive relative to the cathode but which is quite low compared with the operating potential applied to the accelerating electrode. Typically the accelerating electrode is spaced by approximately 0.5 mm from the photo cathode and receives an applied potential of 500 volts positive whereas the second electrode may be spaced by 2 mm for the first mesh electrode and receives an applied potential of approximately 50 volts positive relative to the photo cathode. The purpose of the pair of electrodes is to inhibit secondary emission from the first electrode. The potential applied to the second electrode is preferably as low as possible in order that secondary emission from the second electrode is relatively low. The aforementioned patent specification suggests that the second electrode may be used for gating, a decrease in the potential on the second electrode to below that of the photo cathode cutting off the primary electron beam.
The present invention may employ a tube similar to that described in the aforementioned patent specification but the method of operation and the operating circuit appropriate for performing the present invention differ substantially from the arrangements appropriate for controlling the image tube in the manner described in the aforementioned patent specification.